Field of the Invention
Exemplary embodiments of the invention relate to light emitting diodes and, more particularly, to light emitting diodes having electrode pads.
Description of the Background
Gallium nitride (GaN) based light emitting diodes (LEDs) have been used in a wide range of applications including full color LED displays, LED traffic signals, white light LEDs, etc.
A GaN-based light emitting diode may generally be formed by growing epitaxial layers on a substrate, for example, a sapphire substrate, and include an N-type semiconductor layer, a P-type semiconductor layer, and an active layer interposed between the N-type semiconductor layer and the P-type semiconductor layer. Further, an N electrode pad is formed on the N-type semiconductor layer and a P electrode pad is formed on the P-type semiconductor layer. The light emitting diode is electrically connected to and operated by an external power source through these electrode pads. Here, electric current is directed from the P-electrode pad to the N-electrode pad through the semiconductor layers.
Generally, since the P-type semiconductor layer may have a high resistivity, electric current may not be evenly distributed in the P-type semiconductor layer, but may be concentrated on a portion of the P-type semiconductor layer where the P-electrode pad is formed, and electric current may be concentrated on and flow through edges of the semiconductor layers. This may be referred to as current crowding, and may lead to a reduction in light emitting area, thereby deteriorating luminous efficacy of the light emitting source. To solve such problems, a transparent electrode layer having a low resistivity may be formed on the P-type semiconductor layer so as to enhance current spreading. In this structure, when supplied from the P-electrode pad, the electric current may be dispersed by the transparent electrode layer before entering the P-type semiconductor layer, thereby increasing a light emitting area of the LED.
However, since the transparent electrode layer tends to absorb light, the thickness of the transparent electrode layer may be limited, thereby providing limited current spreading. In particular, in a large LED having an area of about 1 mm2 or more for high output, there may be a limit in achieving efficient current spreading through the transparent electrode layer.
To facilitate current spreading within the light emitting diode, extensions extending from the electrode pads may be used. For example, U.S. Pat. No. 6,650,018, issued to Zhao, et al., discloses an LED which includes a plurality of extensions extending from electrode contact portions, that is, electrode pads, in opposite directions to enhance current spreading.
Although the use of such extensions may enhance current spreading over a wide region of the LED, current crowding may still occur at portions of the LEDs where the electrode pads are formed.
Moreover, as the size of the LED increases, the likelihood of a defect being present in the LED may increase. Defects such as threading dislocations, pin-holes, etc. provide a path through which electric current may flow, thereby disturbing current spreading in the LED.